1. Field of the Invention
The present invention relates to an antenna device for use in a radio communication device such as a mobile radio communication system, local area network (LAN), etc.
2. Description of the Related Art
Linear antennas are known in the art. They include a dipole antenna having conductors with a total length of .lambda./2 (where .lambda. is the resonance wavelength) and a monopole antenna having a conductor with a length of .lambda./4. However, the disadvantage of these linear antennas is their large size. For example, .lambda./4 for an antenna with a resonance frequency of 1.9 GHz is about 4 cm. Such a large value of .lambda./4 results in a large size of the antenna and thus a large size of a mobile communication device.
To avoid the above problem, a dielectric dipole antenna 50 such as that shown in FIG. 10 has been proposed. The dielectric dipole antenna 50 comprises a conductor 52 formed on one principal surface of a dielectric 51 and a conductor 53 formed on the opposite principal surface of the dielectric 51 wherein one end of the conductor 52 extends across a side face of the dielectric 51 and is connected to a feeding terminal 54 formed on the opposite principal surface. One end of the conductor 53 is connected to an electrode 55 formed on the opposite principal surface of the dielectric 51. In this structure, resonance occurs between the conductors 52 and 53. Since the dielectric 51 serves to reduce the wavelength, it is possible to employ shorter conductors for the conductors 52 and 53, thus reducing the total size of the dielectric dipole antenna 50. The dielectric dipole antenna 50 is mounted, as shown in FIG. 11, on a mounting substrate 58 having a transmission line 56 formed on its upper surface and a ground electrode 57 formed on its back surface thereby constructing an antenna device 60. In this antenna device 60, one end of the transmission line 56 is connected to the feeding terminal 54 of the dielectric dipole antenna 50, and the other end of the transmission line 56 is connected to a radio-frequency circuit RF of a radio communication device on which the antenna device 60 is installed. The electrode 55 shown in FIG. 10 is connected to the ground electrode 57 via a via-hole (not shown).
However, in the conventional dielectric dipole antenna described above, although it is possible to reduce the size of the antenna by reducing the physical length of the conductors formed on the principal surfaces of the dielectric, the radiation area decreases with the reduction in the conductor length, and thus a corresponding reduction in the antenna gain occurs. This makes it difficult to reduce the size of the dielectric dipole antenna.
To solve the above problem, a principal object of the present invention is to provide a small-sized antenna device in which the conductor length can be reduced without encountering a reduction in gain.